[Arny Krueger]

"davew" wrote in message


The VU meter is basically a bridge rectifier followed by
a low pass filter. So it's mean rectified, not mean squared.



So a 1dB difference for pure tone. We don't tend to use rms or
mean whatever when talking about audio levels though, we just say
"level" and that seems to be good enough. It's understood that when you
reach 0dBFS you're in trouble shortly thereafter.

http://www.allaboutcircuits.com/vol_2/chpt_1/3.html

" For RMS measurements, analog meter movements (D'Arsonval, Weston, iron
vane, electrodynamometer) will work so long as they have been calibrated in
RMS figures. Because the mechanical inertia and dampening effects of an
electromechanical meter movement makes the deflection of the needle
naturally proportional to the average value of the AC, not the true RMS
value, analog meters must be specifically calibrated (or mis-calibrated,
depending on how you look at it) to indicate voltage or current in RMS
units. The accuracy of this calibration depends on an assumed waveshape,
usually a sine wave. "

IOW, ordinary passive meters are not True RMS meters without a lot of help.

The averaging function is produced mechanically, by the mass of the moving
parts of the meter

If you want to make a passive analog meter that properly reads True RMS, it
must use a movement that responds to the square of the applied voltage, and
is nonlinearly calibrated to indicate the square root of the applied
voltage.

A meter that uses the applied voltage to establish both the magnetic field
around its armature, and also the magnetic field produced by the armature;
will respond to the square of the applied voltage. With this kind of meter,
doubling both voltages (which would ordinarly be based on the same signal)
will interact to produce an attraction or repulsion that quadruples. The
pointer will deflect 4 times further, and the nonlinear calibration of the
scale will provide the square root function.

Most meters use a permanent magnet to establish the magnetic field around
the armature because this makes it easier to produce a sensitive meter. They
will then respond more or less linearly to the intensity of the applied
voltage. Therefore a proper True RMS meter will probably not be the most
sensitive meter around.

Passive True RMS meters were widely used in the days before complex active
electronic circuits became a practical means to accomplish the same end. I
have one that was sold by RCA back in the day for measuring power line
voltage, which has the expected highly nonlinear scale.